Friday, December 20, 2013

By Jared Erickson
The battles between predators and prey across the natural world are punctuated by
vicious physical adaptations on both sides. The preyed upon develop defenses
for discouraging or even attacking back against their predators. The predators
in turn have their own ways to overcome their prey and be sure it is them who
are victorious. A multitude of examples can be found throughout the natural
world. The duck billed platypus of Australia has specialized hind limb spurs
that can be used to stab predators and inject them with a venom that inflicts
excruciating pain, giving the predator a painful lesson instead of a delicious
meal. The majestic lion that is known for ranging across the African savanna
has a set of razor sharp retractable claws it uses to attack and capture a wide
range of prey. The not so cuddly looking
porcupine of North America is covered in a defensive mat of barbed quills that
it can use to puncture and embed in the soft flesh of any predator who thought
to make the porcupine its snack. But vicious and destructive predator and prey
adaptations aren't the sole domain of the animal kingdom; the microbial world
has developed its own.

A single drop of pond water can contain a dizzying array of microbes engaged in
life and death struggles as they try to survive. Examining this tiny liquid
safari finds a myriad of residents including the tiny Paramecium and Didinium.
A unicellular ciliate, Paramecium has a vicious method that it can use
to deter and avoid its carnivorous unicellular predators. This method is
through the use of a system of sharp protrusions known as trichocysts. The Didinium
also lurks about in the water with the Paramecium. But instead of defense,
the Didinium has developed a system for attacking and capturing its prey
through the use of toxicysts. Paramecium live their entire lives swimming about in the water. The beating of the finger
like cilia of the Paramecium propel them through the water. Individual Paramecium
can vary greatly in size, but most are between 100 and 300 µm in total length.
The Paramecium swim from place to place, gathering and feeding on the
bacteria and algae that co-exist in the water. Also lurking about in the water
are the predators of the Paramecium. In addition to the Didinium,carnivorous microbes such as Cephalodella and Eucypris stalk
and attempt to eat the swimming Paramecium. If a predator is able to
strike and capture a Paramecium,it will engulf it whole. The predator will then drift about in the water
as it slowly digests its Paramecium meal.

The Paramecium safely sequesters the trichocysts inside of their own specialized
membrane until the Paramecium feels the need to use them. When that need
arises, a rapid fusion event occurs between the membrane surrounding the
trichocyst, and the Paramecium's cell membrane. This membrane fusion
event violently thrusts the trichocysts outside of the cell, exposing their
sharp spear-like tips in a fraction of a second. The trichocysts extend away
from the cell, attached to the tip of an extension of the membrane, like a spear head on the end of a spear handle.
Trichocysts are sharp and pointed cellular organelles. Shaped like a spearhead, the trichocysts have
a broad base which tapers down into a tiny point. The trichocysts are 3 – 4 µm
in total length. A single Paramecium can harbor thousands of these sharp
trichocysts.

The trichocysts stand as both the best and last line of defense of the Paramecium
from the certain death inside of the predator. When the Paramecium feels
the mouth of the predator, it is able to quickly discharge the trichocysts in
that area. Watching this interaction under a microscope, the predator can be
seen to momentarily recoil from the sharp sting of the trichocysts. This
momentary hesitation on the part of the predator is all the Paramecium
needs to make its escape, furiously swimming away by rapidly beating its cilia.
While the trichocysts are a defense against predation, the Paramecium are not
always so lucky as to escape. The carnivorous Didinium has developed a
system that assists in overcoming the trichocyst defense. Looking at a Didinium
it would be hard to tell that this funny acorn shaped unicellular microbe
is one of the most fearsome hunters of Paramecium. Similar in overall
size to many Paramecium cells, the Didinium is still capable of
swallowing Paramecium whole.

What makes the Didinium particularly dangerous is hidden beneath the tapered
cone shaped proboscis that is located at the anterior end of the cell. The
proboscis is what the Didinium uses to strike at the Paramecium.
When the proboscis makes contact with the surface of the Paramecium, the
Didinium reveals its weapon lurking beneath the surface, the toxicyst.
Toxicysts are similar to the trichocysts, however they are used for offense instead of
defense. The toxicysts rapidly fire from inside the proboscis pierce the Paramecium
and embed themselves firmly in place. Long strands attached to the toxicysts
like ropes attached to a whaler's harpoon keep the Didinium firmly
attached to the Paramecium. The Paramecium can struggle against
the toxicysts and attempt to strike back using its trichocysts, but the Paramecium
will remain firmly in the grasp of the Didinium. With no hope of escape
the Paramecium is drawn in close. The Didinium can then begin to
swallow its immobilized prey whole.

The predator and prey interactions between large animals are familiar to most
people. From film strips and movies in school and Sunday nature programs on
television, we have become familiar with the brutality of the animal world. The
microbial world shows us that taking an expedition to far away lands isn't
required to see carnivorous predators and dangerous prey interact. Down at your
local park in a single drop of pond water, tiny microbes are locked in their
own life and death battles. A simple microscope can allow youto unlock your own microbial safari.

References

1. F. Buonanno, T. Harumoto, C. Ortenzi, The defensive
function of trichocysts in Paramecium tetraurelia against metazoan predators
compared with the chemical defense of two species of toxin-containing
ciliates., Zoolog. Sci.30, 255–61 (2013).

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This blog represents the work of students from one of the few courses devoted to Eukaryotic Microbiology as a whole. This is a new experiment for this course that we hope will be a successful fusion of student learning and science communication. Enjoy, but play nice, comments are welcome but mean-spirited comments will be deleted.